CHAPTER XI 



EFFECT OF CROWDING ON SURVIVAL 

 AND OXYGEN CONSUMPTION 



Studies on the causes and effects of animal aggregations begun in 

 191 2 and carried on intermittently thereafter were summarized 

 (Allee, 1920) for the water isopod Asellus communis, three species of 

 land isopods, and the ophiurid starfish Ophiodcrma hrevispina. The 

 results showed that, in general, bunching in these species is most 

 prevalent under adverse conditions and when there is no means of 

 satisfying the normally positive thigmotactic reaction. Of the single 

 factors tested, the tendency to collect in bunches is most strongly 

 encouraged in Asellus by the breeding reaction, in the land isopods 

 by the amount of moisture present, and in Ophiodenna by the 

 amount of light. 



All of these animals had shown in preliminary experiments a 

 lowered rate of metabolism immediately following the formation of 

 the aggregation, as measured by their oxygen consumption or car- 

 bon-dioxide production. When isolated and bunched animals stand 

 for long periods of time, the effect on the metabolic rate is reversed. 

 Both come to have lower rates than at the beginning of the experi- 

 ment, but the decrease is much greater with isolated than with 

 bunched animals. In the land isopods this decrease is accompanied 

 by a greater loss of water by the isolated individuals. The conclu- 

 sion was drawn that ''under laboratory conditions the formation of 

 aggregations serves to make these animals more quiet and in the 

 long run proves to be what is usually called an adaptive reaction." 

 This early statement remains a fair summary of the evidence since 

 collected in this laboratory. 



In an eariier chapter attention was called to the fact that land 

 isopods aggregate into fairly compact masses in the absence of water 

 and separate when water is added. Experiments on two species, 

 Oniscus asellus and Cylisticus convexus, were run to test the effect of 



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